Because Call of Duty isn't a videogamers Videogame. Call of Duty has been adopted as the go-to game for all of the casuals that will buy and play just a single game. I've been to midnight launchs for a CoD game before. Half the people in that line couldn't tell me how many Call of Duty games there are in the series or even what the name of the console is they'd be playing the game on.
And Population size online doesn't necessarily mean anything. Different people in different time zones play at different times of day. You're talking about players in countries all around the world. 100,000 people at a time might represent a country or two, but that doesn't account for people that are working, sleeping, or otherwise unavailable during the same time on the other end of the planet. And even at only 100,000 players, it's still 4th most active game on Xbox Live. If you go by that standard of judgement, does that mean any game with 100,000 people or less online sucks? So games like Splinter Cell, Ghost Recon, Far Cry 3, and Mass Effect 3 suck? Not mention offline games that don't even generate an online population like Catherine, Dragon Age, Fallout, Skyrim, etc...? The argument of "Only 100,000 people" has no basis for a game being bad.